In the operation of nuclear reactors, it is customary to remove fuel assemblies after their energy has been depleted to a predetermined level. Upon removal, this spent nuclear fuel (“SNF”) is still highly radioactive and produces considerable heat, requiring that great care be taken in its packaging, transporting, and storing. Specifically, SNF emits extremely dangerous neutrons (i.e., neutron radiation) and gamma photons (i.e., gamma radiation).
It is imperative that these neutrons and gamma photons be contained at all times during transfer and storage of the SNF. It also imperative that the residual heat emanating from the SNF be lead away and escape from the SNF to avoid a critical event. Thus, containers used to transfer and/or store SNF must not only safely enclose and absorb the radioactivity of the SNF, they must also allow for adequate cooling of the SNF. Such transfer and/or storage containers are commonly referred to in the art as casks.
Generally speaking, there are two types of casks used for the transportation and for storage of SNF, ventilated vertical overpacks (“VVOs”) and thermally conductive casks. VVOs typically utilize a sealable canister that is loaded with SNF and positioned within a cavity of the VVO. Such canisters often contain a basket assembly for receiving the SNF. An example of a canister and basket assembly designed for use with a VVO is disclosed in U.S. Pat. No. 5,898,747 (Singh), issued Apr. 27, 1999, the entirety of which is hereby incorporated by reference. The body of a VVO is designed and constructed to provide the necessary gamma and neutron radiation shielding for the SNF loaded canister. In order to cool the SNF within the canister, VVOs are provided with ventilation passageways that allow the cool ambient air to flow into the cavity of the VVO body, over the outer surface of the canister and out of the cavity as warmed air. As a result, the heat emanated by the SNF within the canister is removed by natural convection forces. One example of a VVO is disclosed in U.S. Pat. No. 6,718,00 (Singh et al.), issued Apr. 6, 2004, the entirety of which is hereby incorporated by reference.
The second type of casks are thermally conductive casks. In comparison to VVOs, thermally conductive casks are non-ventilated. In a typically thermally conductive cask, the SNF is loaded directly into a cavity formed by the cask body. A basket assembly is typically provided within the cavity itself to provide support for the SNF rods. As with the VVOs, the body of the thermally conductive cask is designed to provide the necessary gamma and neutron radiation shielding for the SNF. In contrast to VVOs, however, which utilize natural convective forces to remove the heat that emanates from the internally stored SNF, thermally conductive casks utilize thermal conduction to cool the SNF. More specifically, the cask body itself is designed to lead the heat away from the SNF via thermal conduction. In a typical thermally conductive cask, the cask body is made of steel, or another metal having high thermal conductivity. As a result, the heat emanating from the SNF is conducted outwardly from the cavity and through the cask body until it reaches the outer surface of the cask body. This heat is then removed from the outer surface of the cask body by the convective forces of the ambient air.
In some instances, the use of VVOs is either not referred and/or unnecessary. This may be due to the heat load of the subject SNF, the existing setup/design of the storage facility at which the SNF is to be stored and/or the nuclear regulations of the country in which the storage facility is located. However, existing designs of thermally conductive casks suffer from a number of drawbacks, including without limitation: (1) less than optimal heat removal; and (2) vulnerability to the escape of substantial radiation (i.e., shine). Additionally, existing methods of manufacture and designs of thermally conductive casks allow little to no flexibility in altering cask dimensions without a total redesign of the cask and/or retooling of the manufacturing facility.